When expressing the continuous tone of a design on a printed matter, the design in print colors on the printed matter is constituted by fine figure groups of dots, lines, and the like, and the shapes and sizes of the fine figures are controlled based on the density of the continuous tone of the design. Continuous tone expression methods include screen continuous tone expression that expresses a continuous tone by fine constituent element groups made of equilateral polygon arrays of halftone dots and the like, and a line tone expression method that forms the outlines, patterns, and the like of a motif (for example, a person, landscape, object, or pattern) in a design by pictorial dots and fine lines.
In the above-described line tone expression method that forms a pattern using pictorial dots and fine lines, the shade (continuous tone) and texture of a motif in a design are expressed using a sketch-like drawing technique. Since the dots and fine lines of the design, for example, intersect and congest, the object shapes and arrangements on the printed matter are complex. The above-described line tone expression method that forms a pattern using pictorial dots and fine lines is therefore a continuous tone expression method capable of reflecting intention in designing. As the main expression technique, when objects are “lines”, the density of the design can be expressed by the object widths and the sparseness of object arrangements. When objects are “dots”, the density of the design can be expressed by the dot sizes and the sparseness of dots.
The typical technique of the above-described line tone expression method that forms a pattern using pictorial dots and fine lines is used to express the tone of a motif in a design like the intaglio-printed image of a banknote. For example, a printed matter having complex objects of dots and fine lines formed by the line tone expression method is hard to duplicate because of the complex object arrangement. In addition, allowing an ordinary person, when trained, to identify during circulation on the market whether the object shapes are authentic by observation using a magnifier or the like, the patterns formed by the method are used all over the world for printed matters such as securities having monetary values. Simultaneously, patterns formed by the line tone expression method add a quality appearance in design, and have therefore been the necessaries of printed matters including securities such as banknotes, stock certificates, and bonds, various kinds of certificates, and important documents (to be referred to as “securities” hereinafter) since old times. FIG. 1 shows an example of the general line tone expression method. In the line tone expression method, main lines 4 and sub-lines 5 exist on a printed pattern 3 of a printed matter 1, as indicated by the partial enlarged view (circle) of the printed pattern 3. In many cases, the main lines 4 have tonal elements. As for the tonal elements of the main lines 4 that express light and shade of a design, the objects of the main lines 4 are thinned to produce lightness or thickened to bring about darkness, thereby expressing a continuous tone. Note that in the example of FIG. 1, the main lines 4 are illustrated as horizontal parallel lines, and the sub-lines 5 as 45° parallel lines. However, the angles of the parallel lines are not particularly limited.
Various techniques are applied to a background pattern called a ground object pattern to produce an anti-counterfeit effect on printed matters of securities. However, along with the recent advent of high-quality color copying machines and computerized color plate making techniques, certificate document counterfeit techniques tend to be rich in variety. Measures against certificate document counterfeit cope with this by growing in sophistication. However, this leads to an increase in manufacturing cost necessary for anti-counterfeit, and for example, introducing dedicated facilities including special machines and tools for obtaining an environment to confirm the anti-counterfeit effect sometimes requires higher cost for authenticity determination.
To determine authenticity of securities such as banknotes, stock certificates, and bonds, various kinds of certificates, and important documents, an anti-counterfeit technique called latent image intaglio has been used for a long time. The latent image intaglio produces the effect by using projecting objects formed by intaglio ink or the like. For example, as indicated by a printed matter 11 shown in FIG. 42, an object group 13 that forms the background of a printed pattern 12 and an object group 14 that forms a latent image are arranged as parallel line objects in two directions with an angular difference of 90°. When the printed surface of the printed matter 11 is observed from the front, it is not easy to recognize the latent image pattern “P”. However, when observing from off-center, as shown in FIG. 43, adjacent projecting objects of the object group 13 formed by, for example, intaglio ink overlap each other at the viewing angle. For this reason, the object group 13 has a low lightness (high density) than that of the objects 14 that form the actual latent image portion. The latent image “P” thus appears as a visible image. In place of the parallel line objects in two directions, objects in one direction may be used by changing the projection height of the intaglio ink (for example, patent reference 1). This technique features easy authenticity determination without using a special discrimination tool.
On the other hand, there exists a useful technique that enables more remarkable authenticity determination using a simple discrimination tool on a printed matter. More specifically, a discrimination tool is overlaid on a printed matter containing an invisible image, thereby making the invisible image visible. The major form of the discrimination tool is a lenticular lens or a transparent sheet (to be referred to as a “parallel line filter” hereinafter) with a parallel line screen printed on it. The techniques of visualizing an invisible image are roughly classified into two types: dot phase modulation and line phase modulation.
As a printed matter whose latent image is visualized upon overlaying a discrimination tool formed from such a parallel line filter and an authenticity determination method therefor, there exists a printed matter having a background image portion printed by parallel line (or halftone dot) objects and a latent image portion printed by parallel line (or halftone dot) objects in a phase different from that of the background image portion. The background image portion and latent image portion of the printed matter appear to be hard to visually recognize discriminately. However, a method is known which allows to visually recognize the background image portion and latent image portion discriminately by overlaying a parallel line filter on the printed matter at a predetermined position.
An example of dot phase modulation includes an image forming method and a printed matter with patterns phase-modulated in the first and second directions, in which a first multi-tone image is formed by overlaying a parallel line filter so as to make the first direction of the printed matter coincide with the parallel line pattern of the parallel line filter, and a second multi-tone image is formed by changing the overlay angle of the parallel line filter so as to make the second direction of the printed matter coincide with the parallel line pattern (see, for example, patent reference 2).
Another example of dot phase modulation is a printed matter in which dots of a dot pattern whose image becomes visible upon overlaying a lens array (for example, flyeye lens, honeycomb lens, or lenticular lens) on the base material include halftone dots in at least two types of screen line numbers and at least two types of screen angles. If the printed matter is authentic, the percent dot area of the dots of the dot pattern does not change. Hence, an invisible image is visualized by overlaying a lens array. If the printed matter is a duplication, the dots reproduced by the size of the screen line number or the halftone dot angle degrade, and the dot density changes. Hence, an image different from the invisible image becomes visible on the printed matter (see, for example, patent reference 3).
An example of dot phase modulation on abroad is Isogram available from Astron Design, Netherlands (see, for example, non-patent reference p. 1340). More specifically, as indicated by a printed matter shown in FIG. 60(a), a flat pattern having an apparently uniform density includes an invisible image formed by the phase of fine halftone dots upon enlargement, as shown in FIG. 60(b). When a dedicated sheet is overlaid on the printed matter, the invisible image is made visible as a negative or positive image, as shown in FIG. 60(c) or 60(d). However, it is impossible to clearly visualize the image because of the flat pattern having a uniform density.
The present applicants have applied for a patent concerning a printed matter using dot phase modulation. This is a latent image printed matter having two latent image patterns formed by periodically arraying a plurality of isochromatic pixels on a base material. The printed matter has a first latent image pattern (invisible image) in a first region where the plurality of pixels are arrayed with a phase shift in the first direction and a second latent image pattern (invisible image) in a second region printed by a functional ink (see, for example, patent reference 4).
An example of line phase modulation is a printed matter having line portions and non-line portions on a base material, in which a plurality of kinds of latent image parallel line patterns each printed in a different color and having a latent image portion formed by shifting the parallel line phase by a ½ pitch with respect to a parallel line pattern having a single pitch and width are superimposed at different angles and printed, and the latent image portion is made visible by overlaying a film having the same pitch as that of the parallel line pattern of the printed matter on the plurality of kinds of invisible images (see, for example, patent reference 5).
An example of line phase modulation on abroad is HIT (Hidden Image Technology) available from Jura, Hungary (see non-patent reference p. 1341). As shown in FIG. 61(a), a flat pattern having an apparently uniform density includes an invisible image formed by the phase of fine parallel lines upon enlargement, as shown in FIG. 61(b). When a dedicated sheet is overlaid on the printed matter, the invisible image is made visible as a negative or positive image, as shown in FIG. 61(c) or 61(d). Note that since it may be possible to identify the invisible image of the printed matter in FIG. 61(a) by normal observation, a visible image is formed as a camouflage pattern by changing the object width of some parallel lines, as shown in FIG. 61(b). Hollow objects may be used to form the visible image. However, when the invisible image is made visible by overlaying the dedicated sheet, the camouflage pattern is simultaneously visualized as a visible image to impede visibility of the visualized invisible image.
In general, a pattern formed by dot phase modulation or line phase modulation is flat.
There has also been proposed an anti-counterfeit image printed matter, in which a unit block is divided into m columns×n rows to form minimum unit blocks b1, b2, b3, b4, . . . having the same shape. Latent images G1, G2, G3, G4, . . . whose pixel units g1, g2, g3, g4, . . . are the minimum unit blocks b1, b2, b3, b4, . . . , respectively, are formed on an image forming sheet. The pixel units g1, g2, g3, g4, . . . are parallel line patterns each formed from one or more parallel lines. One of the different parallel line patterns that are formed from parallel lines at parallel line pitches p including pitches p1, p2, p3, p4, . . . and parallel line angles θ including parallel line angles θ1, θ2, θ3, θ4, . . . constructs the anti-counterfeit image printed matter. Visualizing parallel line sheets obtained by forming, on transparent sheets, different parallel line patterns formed from parallel lines having the same parallel line pitches p and parallel line angles θ as those of the parallel line patterns constructing the pixel units g1, g2, g3, g4, . . . are overlaid, thereby visualizing the latent images G1, G2, G3, G4, . . . (for example, patent reference 6).
The anti-counterfeit image printed matter according to patent reference 6 visualizes the plurality of latent images by changing the parallel line pattern pitch and angle between the unit pixels. However, the visible image can only be expressed as a uniform background pattern. To visualize the latent images, transparent sheets that require pitches and angles conforming to the parallel line patterns of the unit pixels of the latent images are necessary. That is, a plurality of discrimination tools need to be prepared.
In addition, the techniques disclosed in non-patent reference 1 and patent references 2 to 5 cannot completely nonvisualize a latent image, that is, an embedded image. Low affinity to the designs of securities poses the most serious problem because it is hard to apply the techniques to the above-described line tone expression method. Furthermore, the techniques disclosed in non-patent reference 1 and patent references 2 to 5 need an extra printing process in addition to the process of forming a printed pattern by the above-described line tone expression method. This results in an increase in the manufacturing cost necessary for the anti-counterfeit measure.